The present invention relates to improvement in the corrosion resistance of a connecting part for connecting internal components of a lead acid battery.
As the connecting parts for connecting internal components of lead acid batteries such as the strap and the pole, a Pbxe2x80x94Sb alloy containing Sb in an amount of from 2.0 to 4.0% by weight or a Pbxe2x80x94Sn alloy containing Sn in an amount of from 1.0 to 5.0% by weight has been used hitherto. Among these alloys, the Pbxe2x80x94Sn alloy has been mainly used as the connecting part for valve regulated lead acid batteries having a limited amount of electrolyte. This is because Sb causes an adverse effect when it enters into the battery.
The Pbxe2x80x94Sn alloy is a typical eutectic alloy. A Pbxe2x80x94Sn alloy containing Sn in an amount of about 3.0% by weight is formed by lead-rich primary crystals and a crystal structure comprising Sn-rich phase deposited at the grain boundary between the primary crystals. Thus, the Pbxe2x80x94Sn alloy having the foregoing composition can have their primary crystals to grow to a large size. Since the Sn-rich phase deposited at the crystal boundary has a low strength, a cavity (crack) could be produced in the grain boundary at the solidification step.
In the valve regulated lead acid battery, the connecting part for connecting internal components could be corroded. This is attributed to the following oxygen reduction reaction involving the reduction of oxygen gas produced at the positive electrode on a lead part:
Pb+1/2O2+H2SO4xe2x86x92PbSO4+H2Oxe2x80x83xe2x80x83(1)
Since such a lead part is exposed to the exterior of the electrolytic solution, it does not undergo charge-discharge reaction as occurring in the part soaked in the electrolytic solution. Thus, once converted to lead sulfate (PbSO4), this lead part cannot be converted back to lead (Pb). Therefore, the oxygen reduction reaction represented by the foregoing equation (1) causes general corrosion which is uniform corrosion of the surface of the lead part.
In the Pbxe2x80x94Sn alloy, Sn-rich phase deposited at the grain boundary can be easily corroded, causing corrosion to proceed along the grain boundary (grain boundary corrosion). Further, the Pbxe2x80x94Sn alloy is subject to stress developed by the volumetric change of corrosion products that causes the grain boundary to be cracked (stress corrosion cracking). In the worst case, the connecting part would be broken.
As an approach for overcoming the problem that grain boundary corrosion can easily occur due to the growing of crystals of Pbxe2x80x94Sn alloy, there is proposed a method of adding Se, Te, etc. to a Pbxe2x80x94Sn alloy in an unexamined published Japanese patent application No. 9-167611. This method is to add a nucleant such as Se to a Pbxe2x80x94Sn alloy in order to make grains fine and hence solve the problem of grain boundary corrosion and stress corrosion cracking.
However, it was found that even the Pbxe2x80x94Sn alloy comprising the nucleant such as Se has the following problems. That is to say, the alloy comprising finely divided crystal particles is little subject to the progress of grain boundary corrosion but cannot be prevented from general corrosion due to the oxygen reduction reaction. Thus, such a lead part gradually causes reducing in thickness during use in a valve regulated lead acid battery.
Further, a cell connector made of a Pbxe2x80x94Sn alloy comprising large crystals undergoes grain boundary corrosion along the area at which the cell connector is connected, sometimes causing the connecting part to break due to vibration or impact during use. This fracture occurs along the connecting area (final solidification interface). Therefore, this fracture is presumably attributed to the facts that the final solidification interface can easily have an Sn-rich phase thereon and thus can be easily corroded, this Sn-rich phase has a low strength and thus is brittle, and the cell connector is subject to tensile or shearing stress on the connecting area due to vibration, impact or the like. This problem can also occur with a Pbxe2x80x94Sn alloy comprising fine grains including Se or the like incorporated therein. Thus, by this alloy, the strength of final solidification interface has not sufficiently improved.
Accordingly, it has therefore been a great requirement that the Pbxe2x80x94Sn alloy to be used in the connecting part for connecting internal components of a lead acid battery be prevented from grain boundary corrosion, be less subject to general corrosion and have an enhanced strength in itself and a connecting area.
The present invention has the following embodiments.
(1) A connecting part for connecting internal components of a lead acid battery, which comprises a Pbxe2x80x94Sn alloy containing Ag and Se.
(2) The connecting part for connecting internal components of a lead acid battery according to (1) above, wherein the content of Ag is from 0.01 to 1.0% by weight based on the weight of Pb.
(3) The connecting part for connecting internal components of a lead acid battery according to (1) or (2) above, wherein the content of Se is from 0.001 to 0.05% by weight based on the weight of Pb.
(4) The connecting part for connecting internal components of a lead acid battery according to any one of
(1) to (3) above, wherein the content of Sn is from 0.5 to 5.0% by weight based on the weight of Pb.
(5) The connecting part for connecting internal components of a lead acid battery according to any one of (1) to (4) above, which is a strap.
(6) The connecting part for connecting internal components of a lead acid battery according to any one of (1) to (4) above, which is a pole.
(7) The connecting part for connecting internal components of a lead acid battery according to any one of (1) to (4) above, which is a cell connector.
(8) A lead acid battery comprising a connecting part for connecting internal components according to any one of (1) to (7) above.
The first embodiment of implication of the present invention is characterized in that a lead acid battery has a connecting part for connecting internal components comprising a Pbxe2x80x94Sn alloy containing Ag and Se.
In accordance with the present invention, the connecting part for connecting internal components has an enhanced resistance to general corrosion (resistance to conversion to lead sulfate). At the same time, the crystal particles of the lead alloy can be finely divided to disperse Sn-rich phase deposited at the grain boundary, making it possible to improve the resistance to grain boundary corrosion. Further, the enhancement of resistance to grain boundary corrosion can be accompanied by the prevention of the stress corrosion cracking. Moreover, the strength of the alloy can be enhanced, and the castability and weldability of the alloy can be improved.
The present invention is also characterized in that a Pbxe2x80x94Sn alloy to be used in connecting parts for connecting internal components contains Ag and Se in an amount of from 0.01 to 1.0% by weight, and from 0.001 to 0.05% by weight, and Sn from 0.5 to 5.0% by weight, respectively.
Further, the arrangement that the content of Ag, Se and Sn in the Pbxe2x80x94Sn alloy containing Ag and Se fall within the above defined range makes it possible to remarkably enhance both of general corrosion resistance and grain boundary corrosion resistance and hence obtain a high alloy strength. Moreover, in order to exert a more sufficient effect of improving general corrosion resistance, it is more preferred that the content of Ag and Se be from 0.02 to 1.0% by weight and from 0.005 to 0.05% by weight, respectively.